1. Field of the Invention
The present invention relates to a color toner used for performing image formation using an electrophotographic method, and, in particular, to a color toner to be fixed on a recording medium using optical energy from flash light.
2. Description of the Related Art
The electrophotographic method is a technology currently widely used in image formation apparatuses, such as a copying machine, an electrophotographic facsimile, and an electrophotographic printer. As the electrophotographic method, generally a form using a photoconductive insulator is used, as disclosed by U.S. Pat. No. 2,297,691, etc. In this form, an electrostatic latent image is formed, by applying light, such as that of a laser or an LED, on a photoconductive insulator electrified by corona electric discharge or an electric charge supply roller. Then, the above-mentioned electrostatic latent image is developed by a resin powder called toner (called colorant) colored by a pigment or a paint which adheres thereto, and, thus, a visualized toner image is obtained therefrom. Then, this toner image is transferred onto a recording medium, such as a paper or a film. However, since the toner image at this time is only a powder image which is only put on the recording medium, this should be fixed onto the recording medium. Then, as a last process, after fusing the toner on the recording medium by heat, pressure, light, etc., it is solidified, and the toner image fixed onto the recording medium is finally obtained.
Thus, fixing of toner is such as fusing it by a heat, the toner being a powder generally having a thermoplastic resin (referred to as a binding resin, hereinafter) as a main ingredient, and adhering onto a recording medium as mentioned above. As systems for this purpose, a heat roll system in which a recording medium having had a toner image formed thereon is heated and pressurized directly by a roller, and a flash fixing system in which a toner is fixed onto a recording medium by irradiating it with flash light, such as that from a xenon flash lamp, are well known.
The above-mentioned flash fixing system is a system in which optical energy from a glint of light (flash light) of a discharge tube, such as a xenon flash lamp is converted into thermal energy, and, thereby, a toner is fused, and fixed on a recording medium.
As compared with the above-mentioned heat roll system, this flash fixing system has the following features, when it is used in an image formation apparatus:
(1) Since it is non-contacting fixing, the resolution of a toner powder image formed on a photoconductive insulator layer is not degraded thereby.
(2) No warming up time is needed after beginning of power supply, and, thus, quick starting is possible.
(3) Material or thickness of a recording medium, such as a paper with paste, a pre-printed paper, a paper having a different thickness, has no influence on the fixing performance.
A process in which a toner is fixed onto a recording medium by the flash fixing method is as follows:
Flash light emitted from a discharge tube is absorbed by a toner image on a recording medium (powder image), and is changed into thermal energy. Thereby, the toner increases in temperature, is softening and fused, and is stuck onto the recording medium. The temperature falls after irradiation of flash light is finished, the fused toner then solidifies, and is fixed onto the recording medium.
However, a xenon flash lamp generally used, for example, as a discharge tube for the flash fixing, emits light in a range between 400 nm through 1400 nm in wavelength, and especially, has a remarkably a high light emission intensity in a range between 800 nm through 1400 nm of a near-infrared wavelength region, compared with the light emission intensity in the range between 400 nm through 800 nm of visible region. By this reason, it is required that a toner on which flash fixing is performed should have a high absorbance for the light of the near-infrared wavelength region.
However, generally, the binding resin which is the main ingredient of the toner has a very low absorbance for the visible region and the near-infrared region. Moreover, when the colorant is of black, it has a high absorbance for the visible region and near-infrared region. However, when the colorant is of a color toner, such as yellow, cyan, magenta, red, blue or green, it has a some absorbance for the visible region but has a low absorbance for the near-infrared region.
Therefore, it is difficult to properly fix a color toner containing the binding resin and a colorant for a color other than monochrome one, by such flash light as that used for fixing a black toner. Therefore, in order to fix a color toner, it is necessary to apply strong optical energy thereto.
Then, in order to reduce optical energy needed for fixing a color toner on a recording medium by flash light, a technology has been proposed in which an infrared absorbent having an absorbance for a light emission wavelength region of a xenon flash lamp. For example, Japanese Laid-Open Patent Applications Nos. 61-132959, 6-118694, 7-191492 and 2000-147824 disclose making a toner for the flash fixing contain a compound of a family of aminium, a compound of a family of diimonium, or a compound of a family of naphthalocyanine. Moreover, Japanese Laid-Open Patent Application No. 6-348056 discloses a technology of making resin particles containing an infrared absorbent of a family of anthraquinone, a family of polymethine, or a family of cyanine adhere to a toner surface. Furthermore, Japanese Laid-Open Patent Application No. 10-39535 discloses a technology of improving a fixing performance of a color toner with flash light by making tin oxide or indium oxide contain in the toner.
According to the above-described technology, performance of conversion of optical energy into thermal energy is improved by improving fusion performance of the binding resin which is the main ingredient of a color toner by adding the infrared absorbent thereto.
However, the binding resin cannot yet be fused fully only by addition of the above-mentioned infrared absorbent. Moreover, the above-mentioned compound of a family of aminium, compound of a family of diimonium, or the like, preferably used as the infrared absorbent, is colored by itself. Thereby, it may have a bad influence on the saturation, hue or the like of a color image obtained after being fixed onto a recording medium when this is used in a large amount. Therefore, the amount of the infrared absorbent used preferably should be reduced as possible.
As mentioned above, in the related art, in order to fix a color toner positively by flash light, still high optical energy is needed.
The present invention has been devised in consideration of the above-mentioned situation, and, an object of the present invention is to provide a color toner for flash fixing by which formation of a superior printed image can be achieved while aiming at reduction of optical energy required therefor.
A color toner according to the present invention includes, at least, a binding resin, a colorant and an infrared absorbent, wherein the color toner is fixed on a recording medium by flash light, and PAS intensity thereof obtained from integration of a infrared PAS spectrum obtained according to photoacoustic spectroscopy (PAS) analytical measurement through a range between 800 and 2000 nm falls within a range between 0.01 and 0.2 assuming that the value of carbon black is 1.
The above-mentioned photoacoustic spectroscopy (PAS) analyzing method is a method for detecting, finally by a pressure change, a periodic heat change occurring due to application of intermittent light (flash light) on a sample. Measurement by in-situ (spot) is possible by this detecting method.
Specifically, the PAS analyzing method used in the present invention is as follows: When a modulated infrared is absorbed by the sample, the heat corresponding to incident light is generated. Since this generated heat causes a pressure change in a surrounding gas layer, this change is detected by a high sensitivity microphone. Then, a spectrum same as an ordinary infrared spectrum is obtained by carrying out Fourier transform thereof.
As mentioned above, according to the present invention, a color toner having the PAS intensity obtained from integration of the infrared PAS spectrum obtained according to the photoacoustic spectroscopy (PAS) analytical measurement through a range between 800 and 2000 nm which falls within a range between 0.01 and 0.2 assuming that the value of carbon black is 1 has an excellent fixing performance. This was proved by an actual evaluating experimentation which will be described later. Fixing of such a color toner can be positively performed only with a low optical energy equivalent to the energy of the flash light required for performing image fixing only by a black toner in the related art.
In addition, since a light-to-heat conversion efficiency in the infrared region of a color toner having the above-mentioned PAS intensity less than 0.01 is low, sufficient fixing performance is not obtained. Moreover, although a sufficient fixing performance is obtained when the above-mentioned PAS intensity is larger than 0.2, as a large amount of the infrared absorbent is needed for making the PAS intensity thus increase, adverse influence of reducing the saturation of a resulting color image obtained after fixing it, may occur, as mentioned above.
According to the present invention, a color toner by which the optical energy required for flash fixing can be effectively reduced is obtained. Such a color toner can be positively fixed only with a weak optical energy which might provide a poor fixing performance in the related art. Since the amount of the infrared absorbent used can thus be controlled, the clear color image formation without degradation of the color image obtained after fixing can be obtained.
The PAS intensity of the color toner may be preferably set in a range between 0.2 and 0.9 times the PAS intensity of a black toner which is fixed simultaneously therewith.
Thereby, the color toner which is fixed simultaneously with the black toner on the recording medium in an image formation apparatus shows a satisfactory fixing performance. That is, the fixing performance of the color toner and black toner can be made equivalent by making the difference in the PAS intensity between the color toner and black toner fall within a predetermined range.
When the PAS intensity of the color toner is less than 0.2 times the black toner, and flash fixing is carried out with the energy which provides a satisfactory fixing performance for the black toner, the color toner comes to thus be fixed in poor fixing performance. When flash fixing is carried out with the energy which provides a satisfactory fixing performance for the color toner, voids may be generated in the black toner as a result of it being melted much by this superfluous optical energy. Thereby, the image quality may be degraded. Thus, in case the PAS intensity of the color toner is less than 0.2 times the black toner, it is difficult to fix the color toner and black toner simultaneously in a satisfactory fixing condition.
When the PAS intensity of the color toner is larger than 0.9 times that of the black toner, satisfactory fixing performance can be obtained for both the color toner and black toner simultaneously. However, in this case, a very large amount of the infrared absorbent should be added in order to improve the PAS intensity, and thereby, adverse influence of degrading the saturation of the resulting color image or the like may occur.
According to the present invention, even using the color toner simultaneously with the black toner to be fixed on a recording medium, the color toner provides a satisfactory fixing performance.
The PAS intensity of the color toner may be preferably set in a range between 0.2 and 5 times the PAS intensity of another color toner which is fixed simultaneously therewith.
Thereby, the first color toner which is fixed simultaneously with the second color toner on the recording medium in an image formation apparatus shows a satisfactory fixing performance. That is, the fixing performance of the first and second color toners can be made equivalent by making the difference in the PAS intensity between these color toners fall within a predetermined range.
When the PAS intensity of one (first) color toner is less than 0.2 times another (second) color toner, and flash fixing is carried out with the energy which provides a satisfactory fixing performance for the second toner, the first color toner thus comes to be fixed in poor fixing performance. When flash fixing is carried out with the energy which provides a satisfactory fixing performance for the first color toner, voids may be generated in the second color toner as a result of it being melted much by this superfluous optical energy. Thereby, the image quality may be degraded. Thus, in case the PAS intensity of one color toner is less than 0.2 times that of another color toner, it is difficult to fix both color toners simultaneously in a satisfactory fixing condition. On the contrary, when the PAS intensity of one color toner is larger than five times that of another color toner, it is not possible to fix both the color toners simultaneously in satisfactory fixing condition.
According to the present invention, even using the different color toners simultaneously to be fixed on a recording medium, each color toner provides a satisfactory fixing performance.
The infrared absorbent may preferably comprise at least two infrared absorbents having different absorption wavelength spectrums in a range between 800 and 2000 nm in wavelength.
Thereby, the absorbance of a toner in the range between 800 and 2000 nm in wavelength can be improved, the PAS intensity of the color toner can be improved by this, and a satisfactory fixed image can be realized. Problems, such as poor saturation of a fixed image mentioned above occurring when the utilization efficiency of the optical energy of incident light is not satisfactory even when the additive amount of a single sort of infrared absorbent is increased, since the above-mentioned infrared absorbent has an absorption peak only in a specific range. The optical energy utilization efficiently can be improved by using together two or more sorts of infrared absorbents having different absorption wavelength spectrums at once, and the problem of deterioration of the fixing quality of a resulting image by using a single sort of infrared absorbent so much can be eliminated.
Thus, absorbance of toner in the range of 800-2000 nm wavelength can be improved positively, the PAS intensity of the color toner can be improved by this, and a good fixing image can be realized.
Since two or more infrared absorbents are used together, flash light can be more efficiently utilized and optical energy can be reduced effectively. Furthermore, since the amount of the infrared absorbents used can be controlled in total, adverse influence on color image formation for hue or saturation of resulting image can be more positively controlled.
As the composition of the infrared absorbent to be added to the toner, a first infrared absorbent (A) which has an absorption peak in the range between 800 and 1100 nm in wavelength, and a second infrared absorbent (B) which has an absorption peak in the range between 1100 and 2000 nm in wavelength may be used simultaneously.
Furthermore, the above-mentioned object of the present invention can be achieved by an image forming method including steps in which a color toner which at least contains a binding resin, a colorant, and an infrared absorbent is irradiated by flash light, and thus, the toner is fixed onto a recording medium, wherein the PAS intensity (S) of the color toner obtained from integration of an infrared PAS spectrum obtained according to photoacoustic spectroscopy (PAS) analytical measurement through a range between 800 and 2000 nm and an optical energy (E) of the flash light satisfy the requirement of the following formula (1):
0.03xe2x89xa6Exc2x7Sxe2x89xa60.15xe2x80x83xe2x80x83(1)
Thereby, a satisfactory fixing performance can be obtained while few voids are generated in the toner in forming a color printed image.
The above-mentioned formula (1) means that, in case the optical energy of the flash light is sufficiently high, satisfactory fixing can be performed even when the PAS intensity of the color toner is somewhat low, while, a satisfactory fixing performance can be obtained when the PAS intensity is sufficiently high even when the flash light energy is somewhat low.
That is, fixing cannot be performed satisfactorily in case the above-mentioned product Exc2x7S is less than 0.03, while, when it exceeds 0.15, since this means that a superfluous energy is applied to the color toner, voids may be generated therein, and, thus, a satisfactory printed color image cannot be obtained, either. That is, this inventor etc. found out that a satisfactory fixed (printed out) image can be obtained, even with a reduced optical energy, when flash fixing is performed in such a manner that the requirement of the above-mentioned formula (1) be satisfied. In order to realize image formation which satisfies such a requirement, it is preferable to use the above-mentioned color toner according to the present invention.
By using the above-mentioned color toner, the preferable flash-fixing image forming method for fixing a color toner image on a recording medium by exposure by flash light is realizable, and by this method, energy of flash light may be made into a range between 0.5 and 2.5 J/cm2, and the light application time thereof may be set in a range between 500 and 3000 xcexcs. Thus, the stabled fixing performance can be obtained with a low optical energy without generation of voids in the toner, as compared with the related art.